The following background description constitutes a description of the background of the present invention, and need not necessarily constitute prior art.
Many vehicles, e.g. heavy goods vehicles such as buses, trucks, loaders and mining vehicles, are today equipped with systems for both service braking and auxiliary braking. The service brake system typically uses traditional disc brakes and/or drum brakes placed at the vehicle's wheels. The service brake system is used as the vehicle's principal brake system, e.g. braking the vehicle while it is driven. The service brakes provide a very large braking force. However, the service brakes may overheat in case of a longer period of braking, e.g. on long downhill slopes.
The auxiliary brakes may use braking devices which act on the driveline, e.g. a retarder, exhaust braking or engine braking. The auxiliary brakes may e.g. be used when a sustained braking over a long period of time and with no risk of overheating is required. Thus, auxiliary brakes may e.g. be used when braking on long downhill slopes and/or by systems for constant cruise control, where braking is often used during longer periods.
Auxiliary brakes may also be used to relieve the service brake system and thus to reduce wear and tear and service costs for the service brake system. Therefore, auxiliary brakes may often be used at the same time as the service brakes, e.g. in case of brake pedal braking. Auxiliary braking may also be used by different types of cruise controls to avoid that speed limits are exceeded.
In case of auxiliary braking by engine braking, also called dragging, the engine is driven to rotate since the vehicle rolls forward over the road section, while no fuel is supplied to the engine. Internal resistance in the engine that occurs when it is driven to rotate by the rolling, where the internal resistances arise e.g. in case of compressions in the cylinders and due to frictions in the engine, applies a braking force to the driveline during engine braking.
Reinforced engine braking may be obtained by so-called decompression braking, where valves in the cylinders are opened every time the pistons in the cylinders are in the final phase of compression. Decompression braking is thus a method to increase the engine's braking torque at dragging.
Reinforced engine braking may also be obtained by way of so-called exhaust braking. In such auxiliary braking by exhaust braking, one or several dampers in the exhaust treatment system are controlled, which means that the exhaust back pressure increases so that an increased braking action is achieved during dragging. The exhaust treatment system here comprises exhaust pipes in which the exhaust is led from the engine to an exhaust purifying devices in the exhaust treatment system, e.g. oxidation catalysts, filters and/or reduction catalysts. The one or several dampers which are controlled may in this instance e.g. be placed downstream of the manifold and relatively near the manifold. Exhaust braking is thus a method to increase the engine's braking torque during dragging.
In auxiliary braking through a retarder, a rotor is set into motion by the driveline, usually by an output shaft from the gearbox. The retarder also comprises a stator, and a space between the stator and the rotor which may be filled with e.g. oil. The braking force for the retarder is controlled by way of a regulation of the oil pressure in the retarder, so that the braking force increases with an increasing oil pressure. Here, the braking force also acts on the driveline.